In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for radio communication. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to evolve the WCDMA standard towards the fourth generation (4G) of mobile telecommunication networks. In comparisons with third generation (3G) WCDMA, LTE provides increased capacity, much higher data peak rates and significantly improved latency numbers. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation.
LTE technology is a mobile broadband wireless communication technology in which transmissions are sent using orthogonal frequency division multiplexing (OFDM), wherein the transmissions are sent from base stations, also referred to herein as network nodes or eNBs, to mobile stations, also referred to herein as user equipments or UEs. The transmission OFDM splits the signal into multiple parallel sub-carriers in frequency.
The basic unit of transmission in LTE is a resource block (RB) which in its most common configuration comprises of 12 subcarriers and 7 OFDM symbols in one time slot. A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE), as shown in FIG. 1. Thus, an RB comprises 84 REs.
Accordingly, a basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies for subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized sub-frames, #0-#9, each with a Tsub-frame=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
An LTE radio sub-frame is composed of multiple RBs in frequency with the number of RBs determining the bandwidth of the system and two slots in time, as shown in FIG. 3. Furthermore, the two RBs in a sub-frame that are adjacent in time are denoted as an RB pair.
The signal transmitted by the network node in a downlink, that is, the link carrying transmissions from the network node to the user equipment, sub-frame may be transmitted from multiple antennas and the signal may be received at a user equipment that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a user equipment relies on reference signals (RS) that are transmitted on the downlink. These reference signals (RS) and their position in the time-frequency grid are known to the user equipment and hence may be used to determine channel estimates by measuring the effect of the radio channel on these signals.
It should be noted in this context that the channel an user equipment measures is not necessarily from a particular physical transmit antenna element at the network node to the user equipments receiver antenna element, since the user equipment base the measurement on a transmitted RS and the channel it measures depends on how the particular RS is transmitted from the multiple physical antenna elements at the network node. Therefore, the concept of an antenna port is introduced, where an antenna port is a virtual antenna that is associated with an RS.
Hence, a user equipment measures the channel from an antenna port to the receiver antenna element using the RS associated with that antenna port but which or which group of physical transmit antenna elements that are actually used for the transmission of this RS is transparent and also irrelevant for the user equipment; the transmission on an antenna port may use a single physical antenna element or a combination of signals from multiple antenna elements. Hence, in the effective channel that the user equipment measures from the antenna port, the used precoding or mapping to physical antenna elements is transparently included.
An example of utilization of multiple antenna elements is the use of transmit precoding to direct the transmitted energy towards one particular receiving user equipment, by using all available antenna elements for transmission to transmit the same message, but where individual phase and possibly amplitude weights are applied at each transmit antenna element. This is sometimes denoted UE-specific precoding and the RS in this case is denoted UE-specific RS. If the transmitted data in the RB is pre-coded with the same UE-specific precoding as the data, then the transmission is performed using a single virtual antenna, i.e. a single antenna port, and the user equipment need only to perform channel estimation using this single UE-specific RS and use it as a reference for demodulating the data in this RB.
The UE-specific RS are transmitted only when data is transmitted to a user equipment in the sub-frame otherwise they are not present. In LTE, UE-specific RS are included as part of the RBs that are allocated to a user equipment for reception of user data.
FIG. 4 shows examples of UE-specific reference signals in LTE, where for example all RE denoted R7 belong to one “RS”, hence what is known as an RS is a collection of distributed REs comprising reference symbols.
Another type of reference signals are those that may be used by all user equipments and thus have wide cell area coverage. One example of these is the common reference signals (CRS) that are used by user equipments for various purposes including channel estimation and mobility measurements. These CRS are defined so that they occupy certain pre-defined REs within all the sub-frames in the system bandwidth irrespectively of whether there is any data being sent to users in a sub-frame or not. In FIG. 3, these CRS are shown as “reference signals” or “reference signals comprising a set of reference symbols”.
Messages transmitted over the radio link to users may be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each user equipment within the system. Control messages could include commands to control functions such as the transmitted power from a user equipment, signalling of RBs within which the data is to be received by the user equipment or transmitted from the user equipment and so on. Examples of control messages are the physical downlink control channel (PDCCH) which for example carry scheduling information and power control messages, the physical HARQ indicator channel (PHICH), which carries ACK/NACK in response to a previous uplink transmission and the physical broadcast channel (PBCH) which carries system information.
In LTE Release 10, control messages are demodulated using the CRS, except for the R-PDCCH as is seen below, hence they have a wide cell coverage to reach all user equipments in the cell without having knowledge about their position. The first one to four OFDM symbols, depending on the configuration, in a sub-frame are reserved for control information, as shown in FIG. 3. Control messages could be categorized into those types of messages that need to be sent only to one user equipment, that is, UE-specific control, and those that need to be sent to all user equipments or some subset of user equipments numbering more than one, that is, common control, within the cell being covered by the network node.
It shall be noted in this context that in future LTE releases, there will be new carrier types which may not have a PDCCH transmission or transmission of CRS.
PDCCH Processing
Control messages of PDCCH type are transmitted in multiples of units called Control Channel Elements (CCEs) where each CCE maps to 36 REs. A PDCCH may have aggregation level (AL) of 1, 2, 4 or 8 CCEs to allow for link adaptation of the control message. Furthermore, each CCE is mapped to 9 resource element groups (REG) comprising 4 RE each. These REG are distributed over the whole bandwidth to provide frequency diversity for a CCE. Hence, the PDCCH, which comprises up to 8 CCEs spans the entire system bandwidth in the first n={1, 2, 3 or 4} OFDM symbols, depending on the configuration.
In FIG. 5, one CCE belonging to a PDCCH is mapped to the control region which spans the whole system bandwidth.
Introducing an Enhanced Control Channel (eCCH)
Transmission of the physical downlink shared data channel (PDSCH) to user equipments may use REs in RB pairs that are not used for control messages or RS and may either be transmitted using the UE-specific reference symbols or the CRS as a demodulation reference, depending on the transmission mode. The use of UE-specific RS allows a multi-antenna network node to optimize the transmission using precoding of both data and reference signals being transmitted from the multiple antennas so that the received signal energy increase at the user equipment. Consequently, the channel estimation performance is improved and the data rate of the transmission could be increased.
In LTE Release 10, a relay control channel was also defined, denoted R-PDCCH for transmitting control information from network node to relay nodes. The R-PDCCH is placed in the data region, hence, similar to a PDSCH transmission. The transmission of the R-PDCCH may either be configured to use CRS to provide wide cell coverage or relay node (RN) specific reference signals to improve the link performance towards a particular RN by precoding, similar to the PDSCH with UE-specific RS. The UE-specific RS is in the latter case used also for the R-PDCCH transmission. The R-PDCCH occupies a number of configured RB pairs in the system bandwidth and is thus frequency multiplexed with the PDSCH transmissions in the remaining RB pairs, as shown in FIG. 6.
FIG. 6 shows a downlink sub-frame showing 10 RB pairs and transmission of 3 R-PDCCH, that is, red, green or blue, of size 1 RB pair each. The R-PDCCH does not start at OFDM symbol zero to allow for a PDCCH to be transmitted in the first one to four symbols. The remaining RB pairs may be used for PDSCH transmissions.
In LTE Release 11 discussions, attention has turned to adopt the same principle of UE-specific transmission as for the PDSCH and the R-PDCCH for enhanced control channels (eCCH), that is, comprising PDCCH, PHICH, PCFICH, PBCH, by allowing the transmission of generic control messages to a user equipment using such transmissions be based on UE-specific reference signals. This means that precoding gains may be achieved also for the control channels. Another benefit is that different RB pairs may be allocated to different cells or different transmission points within a cell, and thereby may inter-cell interference coordination between control channels be achieved. This frequency coordination is not possible with the PDCCH since the PDCCH spans the whole bandwidth.
FIG. 7 shows an ePDCCH which, similar to the CCE in the PDCCH, is divided into multiple groups and mapped to one of the enhanced control regions. That is, a downlink sub-frame showing a CCE belonging to an ePDCCH that is mapped to one of the enhanced control regions, to achieve localized transmission.
Note that, in FIG. 10, the enhanced control region does not start at OFDM symbol zero, to accommodate simultaneous transmission of a PDCCH in the sub-frame. However, as was mentioned above, there may be carrier types in future LTE releases that do not have a PDCCH, in which case the enhanced control region could start from OFDM symbol zero within the sub-frame.
Even if the eCCH enables UE-specific precoding and such localized transmission, as shown in FIG. 10, it may in some cases be useful to be able to transmit an eCCH in a broadcasted, wide area coverage fashion. This is useful if the network node does not have reliable information to perform precoding towards a certain user equipment, then a wide area coverage transmission is more robust, although the precoding gain is lost. Another case is when the particular control message is intended to more than one user equipment, in this case, UE-specific precoding cannot be used. An example is the transmission of the common control information using PDCCH, that is, in the common search space. In yet another case, sub-band precoding may be utilized, since the user equipment estimates the channel in each RB pair individually, the network node may choose different precoding vectors in the different RB pairs, if the network node has such information that the preferred precoding vectors is different in different parts of the frequency band.
In any of these cases a distributed transmission may be used, as shown in FIG. 8, where the eREG belonging to the same ePDCCH are distributed over the enhanced control regions.
FIG. 8 shows a downlink sub-frame showing a CCE belonging to an ePDCCH is mapped to multiple of the enhanced control regions, to achieve distributed transmission and frequency diversity or sub-band precoding.
FIG. 9 shows a downlink RB pair showing an example with 4 enhanced resource element groups (eREG) each comprising 36 RE, i.e. (42-6 RE), and 2 antenna ports (AP0, AP1). Each eREG is associated with an antenna port and each AP is associated with 2 eREG. Note that according to other examples an eREG may comprise 72 REs.
Thus, one concept for enhanced control signal transmission with UE-specific reference signals is wherein for each configured RB or RB pair used for control channel transmission, multiple orthogonal resources are defined. A resource is most generally defined as a region in the time-frequency OFDM grid comprising a subset of the RE in the RB or in the RB pair plus a cover code from a set of orthogonal cover codes. Hence, the resources are orthogonally multiplexed in time, frequency and code domain, that is, TDM, FDM and CDM, respectively. Below, without loss of generality, it is assumed that the code division is not used, instead a resource is defined as a region in the time frequency grid only.
Each of the time frequency resources is associated with a unique RS, or equivalently antenna port, which is located in the same RB or RB pair. When a user equipment demodulates the information in a given resource of the RB or RB pair, it uses the RS/antenna port associated with that resource. Furthermore, each resource in an RB or RB pair may be independently assigned to user equipments. FIG. 9 shows an example, where time and frequency division of RE into resources denoted enhanced RE groups, that is, the eREG is one resource, is used and where each eREG is associated with one RS from the set of orthogonal RS in the RB or RB pair.
Each eREG is associated with an Antenna Port (AP) and this may, for example, be described with a node diagram as shown in FIG. 10. Here, it may be seen that eREG 1 and eREG 3 are associated with antenna port (AP) 0. When a user equipment demodulates part of an ePDCCH transmitted in for example eREG1, it will use the RS associated with AP0 for demodulation.
FIG. 10 shows the association between AP and eREG in the example shown in FIG. 9. Note that even if multiple orthogonal RS are used in the RB or RB pair, there is only one layer of control data transmitted. As is shown in FIG. 10, it is possible that more than one eREG is using one AP, which is possible since the eREG are orthogonal in the time-frequency OFDM grid. Note that in this case will both eREG1 and eREG3 use the same precoding vector since they use the same antenna port.
The use of antenna ports here shall not be confused with MIMO multiple layer transmission in an RB pair, where each of the multiple RS or AP corresponds to a transmitted MIMO layer. If this would be the case, one eREG would have multiple layers and each eREG would then need to be associated with more than one AP, one per layer. FIG. 11 shows the related node diagram for this case. FIG. 11 shows the association between AP and eREG in the case of spatial multiplexing where eREG 1 contains two layers, each associated with an AP.
In each resource, control information is transmitted comprising, but not limited to, an enhanced PDCCH, a CCE or a fraction of a CCE, an enhanced PHICH or an enhanced PBCH. If the resource is too small to fit a whole enhanced PDCCH, CCE, PHICH or PBCH, a fraction may be transmitted in the resource and the other fraction in other resources in other RB or RB pairs elsewhere in the same sub-frame as was shown in FIG. 11. Note that resources in other RB or RB pairs are associated with their respective antenna ports within the same RB or RB pair.
A typical channel estimator operating on UE-specific RS also use the transmitted common reference signals (CRS) for the tuning of its channel estimation filters, such as, for example, estimating Doppler and coherence bandwidth. The eCCH is intended to be used also on new carrier types, which may not have any CRS transmissions at all.
This means that such channel estimation tuning is not possible or must be performed based on other, eventually more sparse, reference signals. This results in degraded channel estimation performances.